The invention relates to electronic devices, and, more particularly, to photodetectors based on narrow bandgap semiconductor materials and the processing of such materials.
Alloys of mercury telluride with cadmium telluride, which are generically denoted Hg.sub.1-x Cd.sub.x Te, find extensive use as photosensitive semiconductors for infrared radiation detection. Indeed, the two atmospheric windows of greatest interest for infrared radiation detection are at 5-8 .mu.m and 10-12 .mu.m; and Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub.0.73 Cd.sub.0.27 Te has a bandgap of about 0.24 eV which corresponds to a photon wavelength of 5 .mu.m. Both photodiode and photocapacitor detectors are used in detector arrays for infrared imaging.
Frequently, such photodetectors made in a thin film of Hg.sub.1-x Cd.sub.x Te on a lattice-matched wide bandgap substrate such as CdTe or CdZnTe. These materials are all referred to as II-VI compounds because the constituent elements are from Groups II and VI of the periodic table. FIG. 1 illustrates a 50 .mu.m thick film 102 of Hg.sub.1-x Cd.sub.x Te on a CdTe substrate 104. The film may be grown on the substrate by various methods such as liquid phase epitaxy (LPE), metalorganic chemical vapor deposition (MOCVD), molecular beam exitaxy (MBE).
CdTe and CdZnTe and other substrates providing epitaxial support for Hg.sub.1-x Cd.sub.x Te active thin films typically contain fast-diffusing impurities such as copper (Cu) as residual impurities. These impurities normally arise from traces in the starting elements (Cd, Zn, and Te) which are compounded to form the substrate and have typical levels on the order of 1.times.10.sup.15 atoms/cm.sup.3. Such impurities may limit performance of Hg.sub.1-x Cd.sub.x Te thin films on the substrates because the impurities segregate into the thin film (suggested by erratic arrows in FIG. 1) and are electrically active p-type dopants. This may change the doping type in lightly doped n-type films. Indeed, Myers et al., Dopant diffusion in HgCdTe grown by photon assisted molecular-beam epitaxy, 10 J.Vac.Sci.Tech.B 1438 (1992), describe outdiffusion of Cu from such substrates.
Normally, such impurities are gettered to Te inclusions in the substrate. However, substrates without Te inclusions are desired and present a problem for thin film growth, especially by MOCVD and MBE, because of Cu segregation into the epitaxial film. To avoid this problem, higher purity and more costly substrates are required. Alternatively, substrates with minimal Te inclusions can have a pure sacrificial layer of Hg.sub.1-x Cd.sub.x Te grown on them, then be annealed to getter the impurity into the sacrifical layer. Typically, the annealing would be performed at about 400.degree. C. with a Hg.sub.1-x Cd.sub.x Te layer having a melting point of about 800.degree. C. See the Meyers et al. article. Lastly, remove the sacrificial layer. The sacrificial layer can be grown by MOCVD, MBE, LPE, etc., but such an approach is costly in both process complexity and time.
An alternative technique for gettering impurities is solvent extraction, as introduced by Aven and Woodbury in 1962, which anneals II-VI substrates (e.g., ZnS or ZnSe) at high temperatures in Zn melts. FIG. 2 illustrates solvent extraction with II-VI substrate 202 submerged in liquid melt 204; typically the Zn melt will be at about 800-1000.degree. C. Similar solvent extraction has been performed on Hg.sub.1-x Cd.sub.x Te with Hg or Te melts using annealing temperatures in the range of about 300-500.degree. C. This approach frequently leads to heavy surface erosion and surface damage or dislocation slip if solvent is allowed to solidify on the substrate surface. In particular, solvent extraction in a Hg melt produces heavy erosion and is difficult to remove from the surface due to surface tension.
U.S. Pat. No. 4,504,334 describes a method of injecting Hg interstitials into a Hg.sub.1-x Cd.sub.x Te substrate to segregate impurities towards a deposited layer of Te on the opposite side of the substrate for gettering. The gettering layer is in the high vacancy concentration Hg.sub.1-x Cd.sub.x Te region adjacent to the deposited Te and not in the Te layer itself. The process preferably occurs at 280.degree. C. but temperatures upwards of 450.degree. C. are noted.